Friday, 11 May 2012

Boiler water treatment

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Alkalinity

Treatment

For pressures below 20 bar dissolved O2 in the feed does not cause any serious problems so long as the water is kept alkaline
However cold feed should be avoided as this introduces large amounts of dissolved O2 are present, for pressures greater than 18.5 bar a dearator is recommended

All in this column precipitated as hydroxide or phosphate based sludges

All in this column form sodium salts which remain in solution

Sodium Hydroxide

Reacts with highly corrosive MgCl2

Does not readily react with CaSO4

Strongly alkaline

Produces heat when mixed with water

Absorbs CO2 changing to Sodium Carbonate

Unsuitable for standard mixes

Sodium Carbonate Na2CO3 ( soda ash )

Alkaline

At pressures above 14 bar some of the Sodium Carbonate decomposes to form NaOH and CO2 . Increasing on pressure increase

Changes to Sodium Bi-Carbonate when exposed to air

Still usable but larger amounts make control difficult

Standard mix ingredient

Sodium Hexa Meta Phosphate NaPO3 (calgon)

Safe,soluble in water, slightly acidic

May be injected any where as will only react in the boiler

Suitable for LP blrs which require lower alkalinity

DiSodium Phosphate Na2HPO4 (Cophos II)

Neutral used with alkaline additive

Combines with NaOH to give trisodium phosphate

Basic constituent

TriSodium Phosphate Na3 PO4 (Cophos III)

Alkaline

When added to water decomposes to NaOH and Na2 HPO4

As water evaporated density increases and NaOH and Na2 HPO4 recombine

Phosphates can form Phosphides which can coat metal to form a
protective barrier, with excessive phosphate levels, this coating can be
excessive on highly rated boilers operating at higher steaming rates

Chemicals are normally added as a dilute solution fed by a proportioning pump or by injection from pressure pot.
Use of chemicals should be kept to a minimum.
Injection over a long period is preferable as this prevents foaming.
Excessive use of phosphates without blowdown can produce deposits of phosphides on a par with scale formations.
Therefore it is necessary to add sludge conditioners particularly in the forms of polyelectrolytes, particularly in LP blrs

Oxygen Scavengers

Hydrazine N 2 H 4

Oxygen scavenger, continously injected to maintain a reserve within the boiler of 0.02 to 0.1 ppm and a feed water O2 content of less than 10 ppb

At temperatures greater than 350oC , will decompose
to ammonia and nitrogen and will aid in maintaining balanced alkalinity
in steam piping.Steam volatile, neutralises CO2

Inherent alkalinity helps maintain feed water alkalinity within parameters of 8.6 to 9.0.

Used in boiler operating above 32 bar, will not readily react with O2 below 50oC hence risk of copper corrosion occurs with the ammonia stripping off the continuously reforming copper oxides.

Supplied as a 35% solution

Carbohydrazide (N 2 H3)2CO

Is a combined form of Hydrazine

It is superior to hydrazine in performace and is designed to minimise the vapours during handling

Carbohydrazide and its reaction products create no dissolved solids

Is an oxygen scavenger and metal passivator at both high (230'C) and low (65'C) temperatures

Can be used with boilers up to 170 bar

Diethylhydroxylamine DEHA

Like hydrazine, provides a passive oxide film ( magnetite) on metal surfaces to minimise corrosion

Contributes to pH netralisation to an extent that seperate condensate control may not be necessary

Protects entire system-feedwater, boiler and condensate

Sodium sulphite Na2SO3

Takes the form of a soft white powder

Slightly alkaline

Will react with oxygen to form Sodium Sulphate at about 8ppm Sodium Sulphite to 1ppm Oxygen

Use limited to low pressure boilers due to increasing TDS and reducing alkalinity by its action

Tannins

Certain alkaline tannin solutions have a good oxygen absorbing ability with about 6ppm tannin able to remove 1ppm oxygen.

The reaction with oxygen is complex and unreliable, no
official reserve levels exist for the maintenance of a system using
tannin

Erythorbic Acid (Sur-gard) R1-C(OH)

An effective oxygen scavenger and metal passivator

It is the only non-volatile scavenger which can be used with spray attemperation

does not add measureable solids to the boiler water

May be used in boilers up to 122 bar

Officially recognised as a Safe Substance

As with hydrazine a small amount of ammonia is created in the boiler, it is not recommended for layup.

Polymer Treatment

Polymer is a giant molecular built up by stringing together simple molecules
E.G. Polyelectrolytes-Formed from natural or synthetic ionic monomersPolyacrylates - Polymers of acrylic acidPolyamides - Polymers of amides
Polymer treatment prevents scale formation and minimises sludge
formation. It can also loosen scale so established blrs introduced to
this form of treatment may develop leaks where previously plugged with
scale. Especially in way of expanded joints. Also can absorb trace oil
Use limited to LP blrs as no PO4 present to prevent caustic alkalinity
For auxiliary blrs this is a superior form of treatment to the old
alkaline and phosphate treatment. The correct level of alkalinity must
be maintained as too low a level neutralises the electric charge of the
polyelectrolyte. Too high causes caustic alkalinity.

Amine treatment

Compounds containing nitrogen and hydrogen.

Neutralising amines

Hydrazine N2H4

see above

Bramine ( cyclohexalamine )

(Bull & Roberts amine treatment)
Neutralising amine as with hydrazine. Used with hydrazine to maintain
feed water alkalinity within parameters. As a knock on effect will
slightly increase boiler water alkalinity.
Stable at high temperatures so is used more than hydrazine to control
the steam line alkalinity as there is less chance of copper corrosion
which occurs with the prescience of ammonia
Proper boiler water treatment eliminates sludge and scale deposits
within the boiler.
However, over along period of time a film of copper and iron oxides
build up on the tube surface. Most of these oxides are transported from
oxides of corrosion within the feed system to the boiler with the
condensate.
Bramine reduces this corrosion and eliminates the build up of these oxide deposits.Mechanism of function
Condensate from the condenser is very pure and slightly acidic, often
referred to as 'hungry water'. It can dissolve metals in trace amounts
to satisfy this hunger.
Distilled make up water aggravates this situation containing much
dissolved CO2 and hence being acidic carries its own corrosion products.
Trace amounts of bramine are introduced into the system to establish an
alkalinity level greatly reducing the effects of the hungry water.
Some of the bramine is used almost immediately, most however, passes on
to the boiler where it is then transported through boiler water, boiler
stm drum, stm lines back to the condenser. It has no effect anywhere
except the condensate system.
Bramine also has a cleaning effect and may assist in the cleaning the film off the tube over a period of time.
Bramine is safer to handle than Bramine and will protect all metals.
Hydrazine however readily breaks down to form ammonia which whilst
protecting ferrous metals will attack those containing copper

Filming amines

Shows neutralising tendencies, main function however is to coat piping with a molecular water repellent protective film
Injection of amines
May be injected between HP and LP turbines in the X-over pipe or after the dearator.
Adding in X-over pipe-reduces corrosion of copper alloys
Dearator only effective as a feed heater
Adding after dearator -Dearator correctly performing as a dearator and
feed heater.
If possible the best system is to have a changeover to allow norm inj
into the X-over at sea and injection after the dearator when the turbine
shut down

Limits of density/pressure

Sludge conditioning agents

Coagulants-

Mainly polyelectrolytes

Prevents the precipitated sodium based particles forming soft scales

Will keep oil in an emulsion

the water must be kept alkaline

Antifoams

reduce the stability of water film around steam bubble and cause it to collapse.

Common type polyamide is an organic compound of high molecular weight.

In the event of severe contamination separate injection of an antifoam is recommended

8 comments:

I am the author of marineengineering.org.uk which you have copied most of the work on your website against which you are making financial gain. If you do not gain permission or remove it I will take further action. My site is non-commercial, I note you have placed content from marinediesels.info which is a commercial site, you are in breach of his copyright and as such ne may not be as generous in allowing you time to remove his work before taking further action. brian

I am the author of marineengineering.org.uk which you have copied most of the work on your website against which you are making financial gain. If you do not gain permission or remove it I will take further action. My site is non-commercial, I note you have placed content from marinediesels.info which is a commercial site, you are in breach of his copyright and as such ne may not be as generous in allowing you time to remove his work before taking further action. brian

I am the author of marineengineering.org.uk which you have copied most of the work on your website against which you are making financial gain. If you do not gain permission or remove it I will take further action. My site is non-commercial, I note you have placed content from marinediesels.info which is a commercial site, you are in breach of his copyright and as such ne may not be as generous in allowing you time to remove his work before taking further action. brian